1. Field of the Invention
This invention relates to a novel method and apparatus for making a tomogram and particularly to a method and apparatus suitable for making a tomogram of the arrangement of teeth of a human being.
2. Description of the Prior Art
In a typical method of making a tomogram, an X-ray tube is moved so that a long and narrow X-ray beam projected from the X-ray tube through a slit on a film device can move while the beam is intersecting a desired curve, namely, a curve defining the curved surface along which a tomogram is made. In this case, a relative position of the X-ray tube with respect to the film device is kept unchanged. And the film in the film device is moved relatively with respect to the beam so as to allow the beam to scan the film surface. As a result, an X-ray image along the curved surface defined by the desired curve is photographed on the film surface. When the desired curve is a dental arch curve, a tomogram of the dental arch is made on the film surface.
In the orthopantomography well known as a tomography of arrangement of teeth, the X-ray tube device travels along a composite curved path approximate to a parabola joining three arcs. Accordingly, in this case, a curve defining the curved surface along which a tomogram is made is also an approximate parabola. In the orthopantomography, the light and left molar portions each are approximated by one arc and an incisor portion is approximated by another arc. And in an elliptic tomography which is described in detail in U.S. Patent Specification No. 3,806,731, both the X-ray tube and film device are moved along an elliptic path. Accordingly, in this case, a curve defining the curved surface along which a tomogram is made is an elliptic curve.
But there is a substantial difference is dentition between individual persons. Dentition varies with races, sexes or ages, and statistically it is classified into a considerable number of types. For example, there is not a few cases where a dental arch is V-shaped or U-shaped, but irrespective of whether recourse is had to the orthopantomography or to the elliptic tomography in such a case, there is a nonegligible discrepancy between the shape of the path of X-ray tube -- hence the shape of a curve defining the curved surface along which a tomogram is made -- and the shape of an actual dental arch. In making a tomogram such a discrepancy brings about such substantial reductions in the quality of the resulting photograph -- blurring of the photograph -- as hamper a diagnosis or make it difficult.